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Full text of "The manufacture and properties of iron and steel"

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HEJT  TREATMENT.                                    297
carbon alone. It may he compared to a sugar solution which is more or less sweet according to the proportion of sugar present. Marten-site may be easily recognized by its appearance, shown in Fig.XV-B No. 2. At the upper critical point Ar3, the conditions become more favorable for the production of cementite and ferrit'e, and variable amounts of one or the other are formed, depending on the carbon content; at the second critical point, Ar2, no radical change is noticeable, the only effect being an increase in the amount of cementite or ferrite, but at the lower critical point, Arx, the marten-site disappears, and in steels cooled slowly to below this temperature the structure is composed entirely of ferrite, or entirely of pearlite, or of pearlite mixed with ferrite or cementite. Ferrite is iron free from carbon and forms almost the whole of a low carbon steel, while cementite is considered to be a compound of iron and carbon Denoted by the formula Fe3C, the carbon of this form being known as cement carbon. Pearlite is formed by the structural union of ferrite and cementite in definite proportions, not being a compound, but simply an intimate mixture. It appears in two forms, granular and lamellar, the former being seen in steel which has been worked or reheated to a low heat, while the latter is found only in steel which has been cooled slowly through the critical range. It is to the lamellar variety that its name is due, the structure by oblique light giving an effect like mother of pearl. In addition to these common forms there are two others, troostite and sorbite, of which little is known at present. As steel cools through the critical range, the transition from martensite to one of the forms contained in unhardened steel is not abrupt, but appears to be in two steps. Thus by quenching during this critical change a new condition will be obtained—troostite—and if this quenching takes place at the end of the critical range in cooling, a second effect is noticed, which is called sorbite. Quenching in lead, or reheating quenched steel to a purple tint may also produce sorbite, and Osmond states that when small pieces are cooled in air the chilling is sufficiently rapid to prevent the complete transformation into ferrite and cementite, some sorbite being formed. Thus aus-tenite, martensite and troostite are found only in steel quenched at or above the critical range, while ferrite, cementite, pearlite and-sorbite, are characteristic of unhardened steel. It is difficult to'." develop troostite and sorbite in the process of etching in such a way that they will be clearly visible under the microscope, and it has